Somehow water/moisture is getting into your walls. Looks like lime deposits to me.

Beyond that hard to tell. Remember reading in National Geographic, in Venice, they had to insert lead sheeting in the walls to prevent moisture from rising up. Wonder if it's the bottom two feet of walls they intend to replace?

You did not mention what the walls are built of, but I would gues they are some unfired or partiall fired clay brick that is common in India and Pakistan. A second guess would be some kind of a low qualty concrete/pressed dirt block. In either case, the white deposit is called efflorescence, which is formed when water comes to the surface and evaporates and leaves the salt deposit.

I assume most of the walls are interior, but one is obviously a privacy/security wall.

The salt crystals are expansive and can deteriorate the surface (plaster/stucco in your case) and possibly the wall material. The source of the salt could be from the plaster or from the wall materials. Water is the vehicle that dissolves and carries the salt to the surface.

You should get rid of the water in the wall. The source could be absorption and capillary action from below as suggested or from leakage above if you have significant rains. My geography memory is poor, but I do recall being in some significant rains in the south.

In your part of the world, replacing a portion of the middle of a wall is not unheard of. If you do replace, make sure you are not just creating the same problem and situation again. Get several opinions on the suggested repair AND find out what the real cause is, and how they would prevent the same thing from happening again.

Your method of construction is the standard of construction for most of the developed world with minor exceptions (U.S., Canada and maybe Tonga). There are definitely ways to build in your area that will last for centuries.

Without seeing it and getting mor details of the entire home complex, it is difficult to make any more detailed suggestions.

Most of the "fired" clay brick in your part of the world (Pakistan, India and Bangladesh) are not the same as the fired clay brick in other areas. You may have some exceptions.

The reason they are not highly fired (high temperatures, long duration) is your climate (relatively high minimum temperatures) and the needed properties. Also, high temperature firing is only effective with certain types of clay (chenical content) that are not universally available. That is one of the reasons quality concrete products are readily available that quality clay brick products.

The bottom line is to not confuse the suggestion you may get from others that are not familiar with the brick locally available. The high firing has a big effect on the absorption and other properties of the brick.

Rely on a local contractor and/or engineer that can give you some advice based on a knowledge of the local materials and recommended repairs.